Rangeland Management

Forest Operations Equipment Catalog

Carrier Configuration

Introduction

Mechanized forestry equipment, in most cases, evolved from construction equipment. For this reason, many of the machine forms seen in forestry look and function very similarly to construction equipment. Even today, construction equipment is used in forest operations by attaching a forestry attachment to a construction chassis. This is most often done in cases where forestry specific design features are not required.

Forestry specific carriers were designed to address issues with power requirements, terrain conditions, operating conditions, and general functionality. The machine forms used in forestry include the articulated chassis, excavator base, skid steer, compact tracked loader, crane, tricycle, and crawler. This section covers those machine forms used in ground based harvest systems.

Cabs

The United States Occupational Safety and Health Administration (OSHA) defines more stringent protective structures guidelines for cabs on forestry equipment. In addition to Rollover Protective Structures (ROPS), they are required to have Falling Object Protective Structures (FOPS). They are also required to have solid or mesh protection around the cab depending on the location on the cab. This provides additional protection against breaking cables and limbs. See OSHA standards for logging operations (1910.266) for a complete explanation of these cab requirements. Construction equipment that is converted for use in forest operations must be fitted to meet these requirements.

Tracks vs. Wheels

While there is equipment available that operates on legs, the vast majority of ground-based equipment in use operates on either tracks or wheels. The main differences between the two modes of transport are speed and soil effects. Tracked machines are generally slower than wheeled machines. For extraction equipment this translates into lower productivity over longer extraction distances.

Soil effects refer to soil displacement and compaction that is caused during the operation of the equipment. While tracked equipment has more contact with the ground, thus spreading the total weight over a larger area, the vehicle weight tends to be much more than comparable wheeled equipment. This translates into roughly equal ground pressure being exerted by comparably capable wheeled and tracked equipment. The result is little difference in soil compaction between wheeled and tracked machines. Ground pressure can be decreased by installing dual tires or high flotation tires on wheeled equipment.

There is a larger difference in soil displacement between wheeled and tracked machines. Tracks have higher traction because their force is translated through the ground over more surface area. This increased traction allows them to operate on looser and wetter soils without losing traction. The method by which tracked equipment and skid steer wheel loaders turn can increase the amount of soil displacement that occurs. This effect will be more noticeable in partial cuts where movement is limited and tighter turning is required.

Slope by itself limits the ability of equipment to operate due to its affect on stability. Slope considered in conjunction with soil conditions will limit the ability of equipment to operate due to the increased susceptibility to soil displacement and resulting erosion issues, as well as operator safety. In general, a slope of 30% is often suggested as the operating limit for wheeled equipment, whereas the operating limit for tracked equipment is often suggested as 50%. These numbers are dependent on the function being performed by the equipment, the configuration of the machinery (e.g., whether it has a self-leveling cab), and the soil and other conditions on the slope.

Excavator Base

The excavator base is a common machine form that is utilized to perform felling, processing, extraction, and loading functions in the harvest process. The undercarriage consists of two tracks mounted on a frame. The center of the frame houses a large ring gear and bearing on which the chassis is mounted. The chassis contains the engine, cab, and boom. The chassis rotates on the base by means of the ring gear and a hydraulic motor. Turning is accomplished by stopping or reversing the track on one side of the machine.

Forestry specific versions of the excavator base raise the height of the undercarriage frame to accommodate travel over stumps, logging debris, and rough ground. The plates protecting the undercarriage are beefed up to protect the equipment from damage.

Cabs on excavator based forestry equipment are situated higher and closer to the front to increase visibility around the equipment. In addition, additional windows are placed on the top and bottom of the cabs to increase visibility to the tops of trees and the ground. Cabs are also required to meet the OSHA guidelines mentioned above. The engine compartments also receive increased guarding from falling objects.

Another unique feature that was developed for forestry operations was the concept of zero tail swing. Zero tail swing means that the entire chassis and boom rotates within the dimensions of the tracks. This prevents the chassis or boom from hitting trees while it rotates and allows the machine to operate in partial cuts without damage to residual trees. To achieve this, the boom is designed differently from construction booms. This design places the hydraulic hoses and cylinders under the boom, protecting those parts from falling objects.

Some excavator based forestry equipment is built with self-leveling cabs. This allows the cab to maintain a level stance which decreases operator discomfort and shifts the center of gravity when operating on slopes to permit safer handling of trees.

Tracked equipment generally benefits from decreased ground pressure that minimizes soil compaction. They also have a high breakout force, which means that they can navigate steeper slopes and looser soils without breaking traction. Another advantage of tracked equipment is that they can reach more areas of the stand without traveling over the same amount of ground. This limits the amount of soil disturbance and compaction that occurs in the stand.

Articulated Chassis

An articulated chassis has a pivot point around which the chassis rotates. Each section of the chassis has at least one set of wheels. Steering is accomplished by rotating the chassis instead of the wheels themselves. Four-wheel drive loaders, wheeled skidders, harvesters, forwarders, wheeled feller-bunchers, and clambunk skidders use this chassis design. This design allows for a tight turning radius and increased maneuverability within the stand.

As evidenced by the large number of forestry machines that use this configuration, it is a very versatile form. The advantages are a robust design, good maneuverability, and good stability. Most of the designs are wheeled machines which are capable of faster ground speeds than the tracked machines.

Some machines that use this form, such as forwarders, clambunk skidders, and harvesters may have more than one set of wheels on each articulation. This permits the use of tracks that can be slipped on over the wheels. These tracks decrease the ground pressure and increase the traction, making them comparable to tracked equipment in their ability to decrease soil disturbance. Machines without this capability tend to be more susceptible to causing soil disturbance.

Crawler Chassis

The crawler chassis is a tracked configuration that places the cab and engine on the frame with the tracks. Unlike the excavator chassis, the engine and cab do not rotate on the track frame. This configuration is used by tracked skidders and purpose built masticators.

Like the excavator chassis this design is capable of operating on very steep slopes due to its high traction, low flotation, and low center of gravity.

Crawler chasses have low ground speeds when compared to wheeled skidders, and thus are usually reserved for areas where slope and/or tree size requires their use.

There are two types of track configurations that are used in forest operations: rigid-frame tracks and flex tracks. Rigid-frame tracks are the type of track used by crawler dozers. The track drivers bear the load of crawler on the ground. The drivers on flex track crawlers are suspended and wheels between the front and back drivers carry the load. These wheels are allowed to move up and down independently which permits the track to conform to irregularities in the ground. This feature of flex tracks means that traction is maintained when traversing rough ground limiting slope disturbance and compaction.

Skid Steer

Skid steers are small pieces of equipment with two sets of wheels and a cab and engine compartment placed low between the tires. They turn by reversing or stopping the wheels on one side of the machine, just like tracked equipment. This method of turning permits the machine to have a zero turn radius. They have a boom mounted in the front that is capable of mounting a variety of attachments. These machines were developed originally for operating in barns for the agricultural industry but have grown in popularity due to their low cost and high versatility. The most common uses in the forest industry are as feller bunchers, masticators, and brush cutters.

They tend to have a narrow stance and short wheelbase that limits their use on slopes due to the danger of rollover. Their small wheels and style of turning also increases their susceptibility to causing soil disturbance. They are a poor option for working in broken, rough terrain due to their short wheelbase.

Their small size allows them to work among closely spaced trees. This small size does limit the size of trees they can handle.

Compact Tracked Loader

The compact tracked loader is essentially a skid steer with tracks. The advantage is higher flotation and increased traction. This permits them to operate in loose soil conditions with less chance of soil disturbance. They still suffer from a short track and narrow stance which limits their use on slopes.

Tricycle Configuration

Loaders and feller bunchers use the tricycle configuration. It consists of two forward drive wheels and one wheel mounted under the rear of the chassis. They turn by reversing the wheel on one side of the machine. The rear wheel simply rotates 360 degrees in whichever way the machine is going. These are relatively fast, highly maneuverable machines.

They suffer from the fact that they have lower stability, so are limited to low slopes and relatively even ground.

These can be highly productive machines on even ground and low slopes.